Patent GB-866943-A (GB)
Title: Improvements in or relating to electric circuit arrangements
Inventor: CARNT PETER SWIFT
IPC: H04N9/72
Filed: 1958-06-12
Abstract: 866,943. Automatic phase control systems. GENERAL ELECTRIC CO. Ltd. June 9, 1959 [June 12, 1958], No. 18787/58. Class 40(5) [Also in Group XXXVI] An electric circuit comprises first and second D.C. restoration circuits to both of which an input signal is applied so that the circuits operate thereon in opposed senses, and a fixed reference potential is applied to the first D.C. restoration circuit; the output signal of which is integrated and applied as reference potential to the second D.C. restoration circuit; means being provided for integrating the output signal of the latter. An input signal is applied over capacitance 22 (Fig. 2) and amplifier 23 to the cathode of D.C. restoration diode 29 whose anode is grounded and which is shunted by high resistance 30; the cathode being coupled over integrating resistance 31 and capacitance 33 to the cathode of a second D.C. restoration diode 32 which is shunted by high resistance 34 and whose anode is connected over capacitance 35 to the cathode of diode 29 and over integrating resistance 36 and capacitance 37 to the output. A positive going input pulse train produces a corresponding train of narrow negative going pulses 38 of amplitude V (Fig. 3a) at the anode of 23 and at the cathode of diode 29 a train of positive going broad pulses 39 (Fig. 3b) whose width is equal to the period between pulses 38 and whose negative peaks are locked to earth potential. The integrated amplitude at the cathode of diode 32 is AV where A is constant, and the output derived from the anode thereof is a train of negative going pulses 40 (Fig. 3c) of width equal to that of pulses 38, whose positive peaks are locked to a potential of +AV. The final integrated output has positive amplitude (2A-1)V and follows the polarity of the input signal. In a modification the diodes may be respectively reversed, and their anodes may be interconnected by a resistance, so that the output polarity is opposed to that of the input signal. In an application (Fig. 1) a colour television receiver comprises aerial, RF amplifier, frequency changer, IF amplifier and video detector 1 to 5 producing a monochrome signal Y fed through delay line 16 to the cathodes of the electron guns of a three colour C.R. tube 15, and other chrominance signals amplitude modulated on a pair of sub-carriers which pass through bandpass amplifier 6 to synchronous detectors 7, 8 whose demodulation axes are such that their outputs represent (R-Y) and (B-Y) where R, G, B are voltage amplitudes corresponding to red, green, and blue picture content. The detectors are controlled by a reference oscillator 9 energizing 7 directly and 8 over a phase shifter 10, and a combining circuit 11 develops voltages representing (R-Y), (B-Y) and (G-Y) which after amplification are applied to the grids of the colour tube guns. The phase of the chrominance sub-carriers are retarded 33 degrees and a synchronizing signal comprising a sinusoidal oscillation pulse is modulated once per line on the composite signal, 147 degrees advanced in phase with reference to a selected subcarrier. When the phase of oscillator 10 is set to a specific value relative to the sinusoidal oscillation pulse, a negative going synchronizing pulse appears in the output of detector 8 while no synchronizing signal appears in the output of (R-Y) detector 7. A gate circuit 17 selects during synchronizing periods the (R-Y) signal on the appropriate gun for amplification and energization of a circuit 19 of the kind shown in Fig. 2, whose unindirectional output depends in magnitude and polarity on the magnitude and polarity of the input signals; the output operating a reactance valve 21 through lowpass filter 20 to adjust the phase of oscillator 9 so that zero synchronizing signal is produced in the output of the (R-Y) detector 7; the circuit 19 being responsive to either polarity.